Plasma amino acid levels change markedly after the ingestion of a protein meal. An in vivo Plasma Amino Acid (PAA) ration procedure has been developed which demonstrates that the relative changes of the free amino acids in blood after the ingestion of a dietary protein meal reflect the amino acid adequacy of the dietary protein. This rapid in vivo procedure has been adapted to obtain data which is proposed as the ideal dietary amino acid pattern for normal adult man. This pattern is somewhat different from the generally accepted amino acid requirements for man as determined by the cumbersome nitrogen balance technique. However, it is impossible to determine which pattern best defines man's amino acid needs. To be able to evaluate the validity of the amino acid requirements data obtained by the PAA ratio procedure, it will be necessary to adapt this method at a micro scale to the rat. The initial difficulties encountered in adapting the PAA ratio procedure to the rat have now been overcome by employing an orbital sinus bleeding method to allow trauma-free bleeding, and a semi-forced-feeding technique to control the eating time. Utilizing these modifications, the micro PAA ratio procedure has been used to determine the ideal dietary amino acid needs for the 3-month-old rat. The requirement data thus obtained are somewhat different from the generally accepted amino acid requirements as determined by nitrogen balance techniques. By using the PAA ratio method, the amino acid requirements for the one-, two, six-, and twelve-month-old rat will be determined. After these data are complete, long-term growth and development studies will be run to determine whether the rapid in vivo PAA ratio method or the difficult and cumbersome nitrogen balance procedure yields the most accurate and reliable data with respect to amino acid requirements. Success in these studies will provide a rapid, in vivo, plasma amino acid technique to determine amino acid requirements for individuals or large population groups under normal or stress conditions.